Electro Turnkey PCB Assembly SMT DIP
Selection and design of surface mount components is a key segment, the overall design of the product designers in the system structure and the detailed circuit design phase, determine the electrical components performance and functionality, in SMT design phase should be according to equipment actual circumstances and technology and the requirement of the overall design to decide surface mount components encapsulation form and structure. Surface mounted solder joints are both mechanical and electrical connection points, and reasonable selection has a decisive impact on improving PCB design density, productivity, testability and reliability.
There is no difference between the function of surface mounted component and inserted component. The difference lies in component package. Surface mounted packages must undergo high temperatures during welding, and their components and PCBs must have a matching coefficient of thermal expansion.These factors must be taken into account in product design.
The advantages of choosing appropriate packaging are as follows :1).Save PCB area effectively; 2).Provide better electrical performance; 3).Protect the internal components from humidity and other environmental effects; 4).Provide good communication links; 5).Help heat dissipation and facilitate transmission and testing.
Selection of surface mounting components
Surface mounted components are divided into active and passive components.The gull wing and "J" are classified according to the pin shape.The following classification explains the selection of components.
Passive components include single chip ceramic capacitors, tantalum capacitors and thick film resistors in rectangular or cylindrical shape. Cylindrical passive device is called "MELF". It is easy to roll when re-flow soldering is adopted. Special pad design is required and should be generally avoided. Rectangular passive components are called "CHIP" components. They are small in size, light in weight, have good impact resistance, shock resistance and parasitic loss, and are widely used in various electronic products.In order to obtain good weldability, must select electroplating of nickel bottom barrier layer.
There are two main types of surface mounted chip carriers: ceramic and plastic.
The advantages of ceramic chip packaging are as follows: 1)good air tightness and good protective effect on internal structure; 2)short signal path and obvious improvement of parasitic parameters, noise and delay; 3)reduce power consumption.The disadvantage is that no pins absorb the stress produced when the solder paste is dissolved, and CTE mismatch between the package and PCB can lead to weld spot cracking during welding.The most commonly used ceramic chip carrier is the lead-free ceramic chip carrier LCCC.
Plastic packaging is widely used in military, civilian products production, with good cost performance.Its packaging form is divided into:small shape transistor SOT; Small shape integrated circuit SOIC; Plastic with lead chip carrier PLCC; Small J package; Plastic flat package PQFP.
In order to effectively reduce the PCB area, SOIC with the number of pins below 20 and PLCC with the number of pins between 20 and 84 are preferred under the same device function and performance, and PQFP with the number of pins greater than 84 is preferred.